This invention relates generally to gas furnaces and, more particularly, to control of excess air in a gas furnace having a variable speed inducer motor.
In the operation of a gas-fired furnace, combustion efficiency can be optimized by maintaining the proper ratio of the gas input rate and the combustion air flow rate. Generally, the ideal ratio is offset somewhat for safety purposes by providing for slightly more combustion air (i.e., excess air) than that required for optimum combustion efficiency conditions. In order that furnace heat losses are minimized, it is important that this excess air level is controlled.
Since the pressure drop across the heat exchanger is proportional to excess air, it is maintained at a predetermined constant level for a given gas input rate. One method of maintaining such a constant pressure drop is shown in U.S. patent application Ser. No. 802,273 filed on Nov. 26, 1985 now abandoned by the assignee of the present invention. In that application, sensors are provided at the inlet and the outlet of the heat exchanger, and a pressure transducer is provided to receive signals from those sensors to calculate a pressure drop signal which is then provided to the furnace control to responsively vary the speed of the inducer motor so as to maintain a constant pressure drop and thereby maintain the excess air at a constant level. One of the problems with the use of such a transducer is its relatively high cost. Further, the reliability of such a transducer was found to be less than that desired because of apparent thermal instabilities.
It has become common practice in gas-fired furnaces to provide for two different firing stages where each stage has its own gas input rate. Two speed operation can be accomplished with a fixed rate, two speed motor to drive the draft inducer motor and blower motor; however, the electrical consumption of such motors limited to two speeds while operating at low speed would be significantly greater than that of a variable speed electronically commutated motor (ECM), for example. Further, since the inducer motor would operate at only two fixed speeds, the system could not adapt to variable operating and system conditions such as, for example, a variable length of vent system, such that the level of excess air could not be controlled to the degree desired unless the system was tuned for the particular installation.
It is therefore an object of the present invention to provide an improved method and apparatus for controlling the excess air in a gas-fired furnace without the need for field tuning the combustion system.
Another object of the present invention is the provision in a gas-fired furnace for controlling the level of excess air without the use of a pressure transducer.
Yet another object of the present invention is the provision in a gas-fired furnace for controlling the level of excess air in a manner which takes into account the use of variable length vents.
Still another object of the present invention is the provision for controlling a variable speed motor so as to maintain desirable levels of excess air when operating with either a single or multi-stage system.
Yet another object of the present invention is the provision in a gas-fired furnace for an excess air control system which is economical to manufacture and effective in use.
These objects and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.